1. Field of the Invention
The invention relates to an apparatus for liquefying a thermoplastic plastic, in particular an adhesive on the basis of polyurethane.
2. Description of the Prior Art
Such a liquefying apparatus is known from DE-PS No. 2,836,545 and comprises a supply container for the thermoplastic plastic, a heating means for heating the plastic disposed in the supply container and a collecting trough for the liquefied plastic disposed beneath the supply container; from the collecting trough the liquefied plastic is supplied for example by means of a metering pump to an applicator head.
This liquefying apparatus is adapted in particular to processing hot-melt adhesive which is present in the form of a granulate, i.e. the starting granulate can easily be introduced from above into the supply container, heated and thereby liquefied and thereafter processed.
Problems occur when liquefying poorly flowing highly viscous thermoplastic plastics, that is highly viscous sealing compositions, butylene or bonding hot-melt adhesives, in particular adhesives on the basis of polyurethane, because such poorly flowing highly viscous masses cannot readily be poured from above into the supply container.
So-called "drum melters" have therefore been developed, i.e. heating means which are introduced into the opened transport container for such a thermoplastic plastic and heat the highly viscous mass so that it becomes flowable.
However, it is not possible to use such a drum melter to process thermoplastic plastics which react very rapidly with the oxygen present in the air and in particular harden. This applies very much to adhesives on the basis of polyurethane so that when processing such compositions with a conventional drum melter problems are continuously encountered.